red halo in stacked image

Seems like light pollution, is that a possibility in your location? Or, the flats are not correctly applied, can you see (in the list of files below) if MF is added to the filename after calibration?

@vincent-mod

I'm in a bortle 9 location so light pollution certainly exists.

However, I usually manage to eliminate it.

MF was applied to the subs.

what do you mean by "flats are not correctly applied"?

I was wondering whether a white balanced light source is needed for the flats?

Thanks 

I used a narrowband filter

Flats may not be correctly applied, not saying they are, could just be a cause of what you're seeing. However, when it indeed is light pollution, you can use the light pollution tool that should be able to take care of that nicely. Also, when using a narrowband filter, you need to set APP to extract that data properly, in tab 0 you can select to "extract Ha" for instance. This will produce an Ha, mono image, straight from the OSC data.

When you say halo, are you taking about the ring around the bright star, or are you taking about the redness in the image that seems to appear beyond the nebula, around it? If you are referring to that redness as the halo, I would say you need to do a better job cropping your image first. You can see stacking artifacts around the edges. Crop your image first, then run the LP removal and you will see much better results.